1. Field of the Invention
The present invention relates to an optical low pass filter and an image pickup apparatus having the optical low pass filter.
2. Related Background Art
Presently, an image sensor having 5,000,000 or more pixels has been developed, and this high performance image sensor has been employed in a digital camera. However, there is a case where a false color or Moiré fringes are mixed in an obtained image, and there are many problems of the comprehensive image quality to be solved.
In general, in the image pickup system using the solid-state image pickup element having a discrete pixel structure, image information is optically spatially sampled to obtain an output image. In this case, when a high spatial frequency component that is equal to or higher than a sampling frequency is contained in an object, a false signal of a structure, a hue, or the like which is different from that of the object is generated.
In other words, a frequency component that cannot be obtained by the image pickup system (a frequency component that exceeds a Nyquist frequency) cannot be reproduced as image information, which causes a so-called wave shape distortion, the Moiré fringes, the false color, or the like to be formed. For that reason, up to now, an optical low pass filter is disposed in a part of the image pickup system to restrict the high spatial frequency component of the object.
There have been proposed various types of optical low pass filters and the image pickup systems having the optical low pass filters (refer to Japanese Patent Application Laid-Open No. H09-185019 and Japanese Patent Application Laid-Open No. H08-015646).
The false color and the Moiré fringes are classified into a nose that is called “aliasing noise”. The aliasing noise occurs when a high frequency component of an image that has been focused by a lens system interferes with a sampling pattern of the image sensor.
The normal noises can be suppressed to some extent by taking a statistical property into consideration, but there are many cases where the aliasing noise globally occurs depending on the object, and therefore it is difficult to apply a countermeasure against the normal noises to the aliasing noise.
An optical low pass filter is generally employed to suppress the aliasing noise. The optical low pass filter suppresses the amplitude of a frequency component which is higher than the Nyquist frequency which causes the aliasing noise.
An optical element having birefringence such as a crystal plate, or phase diffraction gratings are employed for the optical low pass filter. Among those elements, the optical element using the birefringence is frequently employed because a relatively precipitous cutoff character is obtained.
On the other hand, as another system for suppressing the aliasing noise, there has been proposed a system in which a microlens array is disposed immediately in front of the image sensor, and the lens configuration of the microlens array changes according to the pixel position (OplusE, Vol. 19, No. 9 (September, 1997)).
In this system, light beams that are subjected to defocus differently in each of the positions of a light receiving element enter the respective light receiving elements (the respective pixels) of the image sensor, to thereby realize an excellent MTF. In this system, although the noises occur, the excellent cutoff characteristic is obtained as compared with the normal microlens array.
FIG. 2 shows an MTF 111 of an optical low pass filter using the birefringence. The optical low pass filter using the birefringence is not an optical element that perfectly suppresses the components with frequency of the cutoff frequency fc or higher, and transmits a fair amount of the components. For that reason, in the case where the object has the frequency component that is equal to or higher than the cutoff frequency fc, the Moiré fringes dominantly appear. Also, the system proposed by OplusE, Vol. 19, No. 9 (September, 1997) cannot obtain the sharp cutoff characteristic as in the birefringence system because the proposed system is based on the MTF of the lens array.